1. Field of the Invention
The invention generally relates to memory technology. In particular, the invention relates to non-volatile magnetic memory.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. For example, after a conventional personal computer is powered off, the volatile memory is typically reloaded through a boot up process upon a restart. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data for long periods of time when power is removed: Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), flash memory, and the like. Disadvantageously, conventional nonvolatile memories are relatively large, slow, and expensive. Further, conventional nonvolatile memories are relatively limited in write cycle capability and typically can only be programmed to store data about 10,000 times in a particular memory location. This prevents a conventional non-volatile memory device, such as a flash memory device, from being used as general purpose memory.
An alternative memory device is known as magnetoresistive random access memory (MRAM). An MRAM device uses magnetic orientations to retain data in its memory cells. Advantageously, MRAM devices are relatively fast, are nonvolatile, consume relatively little power, and do not suffer from a write cycle limitation. There are at least three different types of MRAM devices, including giant magneto-resistance (GMR) MRAM devices, magnetic tunnel junction (MTJ) or tunneling magneto-resistance (TMR) MRAM devices, and pseudo spin valve MRAM devices. GMR MRAM devices separate at least two ferromagnetic layers with a metallic layer. In a MTJ MRAM device, at least two ferromagnetic layers are separated by a thin insulating tunnel barrier. A pseudo spin valve MRAM device uses an asymmetric sandwich of the ferromagnetic layers and metallic layer as a memory cell, and the ferromagnetic layers do not switch at the same time.
One problem that has plagued conventional MRAM devices is relatively low write selectivity. In a conventional MRAM device, the magnetic field applied to a selected memory cell is relatively weakly coupled to the selected cell. This undesirably results in relatively high currents to generate a magnetic field, which is used to write to the cell. The required amounts of current can be higher than desired for relatively high-density integrated circuits. In addition, the magnetic field applied to the selected cell can be undesirably coupled to another cell. This can undesirably cause the contents of an unselected cell to be overwritten.
Embodiments of the invention advantageously solve these and other problems by providing improved write selectivity.
The invention relates to methods and apparatus that allow data to be stored in a memory cell, such as a giant magneto-resistance (GMR) cell or a tunneling magneto-resistance (TMR) cell, of a magnetoresistive random access memory (MRAM). Embodiments of the invention advantageously wind a word line around the memory cell to increase the magnetic field induced by the word line. The word line can be formed by connecting a segment in a first layer to a segment in a second layer with the memory cell disposed between the segments in the first layer and the second layer. Advantageously, embodiments of the invention can be fabricated with relatively narrow memory cells and relatively narrow sense lines or bit lines. Embodiments of the invention exhibit relatively high write selectivity, and use relatively low word currents to store data. In one MRAM, current is selectively passed through a word line by allowing current to flow through a corresponding word row line and a corresponding word column line. The word row line and the word column line are also referred to in the art as a word line and a digit line, respectively.
One embodiment according to the invention includes a magnetic memory cell. The magnetic memory cell includes a cell body that stores data in a stored magnetic orientation. The cell body can correspond to a broad variety of giant magneto-resistance (GMR) types including a spin valve mode or a pseudo spin valve mode and can also correspond to a tunneling magneto-resistance (TMR) cell body. The cell body exhibits a first resistance when the stored magnetic orientation of a hard magnetic layer is aligned or is parallel with a magnetic: orientation of a soft magnetic layer and a second resistance when the stored magnetic orientation is anti-parallel with the magnetic orientation of the soft magnetic layer. The magnetic memory cell further includes a conductive word line that forms at least part of one turn of a winding around the cell body. The conductive word line is electrically coupled at a first end to a word row line. At a second end, the conductive word line is electrically coupled to a word column line.
Another embodiment according to the invention includes a conductive word line for a magnetic memory cell, such as a giant magneto-resistance (GMR) cell. The conductive word line includes at least a first segment in a first layer and at least a second segment in a second layer. The first layer and the second layer are arranged relative to a cell body such that the cell body is disposed between the first segment and the second segment. The conductive word line further includes at least one plug, which electrically connects the first segment to the second segment. The first segment and the second segment form an angle of at least 10 degrees at the plug with respect to each other. The first segment can connect to a word row line or to a word column line.
Another embodiment according to the invention includes a magnetoresistive random access memory (MRAM). The MRAM includes a plurality of memory cell bodies, such as a plurality of GMR cell bodies, where a memory cell body stores a magnetic orientation to represent a logic state and has a first resistance when a stored magnetic orientation in a hard layer is aligned or is in parallel with a magnetic orientation in a soft layer, and a second resistance when the stored magnetic orientation in the hard layer is anti-parallel with the magnetic orientation of the soft layer. The memory cell bodies are arranged in an array. The MRAM further includes a plurality of word lines, where a word line wraps at least 270 degrees around a corresponding memory cell body. The MRAM includes a plurality of word row lines, where a word row line is electrically coupled to a first end of word lines of multiple memory cell bodies along multiple bit lines, such as sense lines, and the MRAM further includes a plurality of word column lines, where a word column line is electrically coupled to a second end of word lines of multiple GMR cell bodies corresponding to the same bit line. A word line for a particular memory cell body is energized by activating a corresponding word row line and a corresponding word column line.
Another embodiment according to the invention includes a magnetoresistive random access memory (MRAM). The MRAM includes a plurality of magnetic memory cell bodies, such as GMR cell bodies or TMR cell bodies, that are coupled to a plurality of bit lines. Word lines in the MRAM wrap individually wrap around cell bodies. Only one cell body per bit line is wrapped by a word line.
Another embodiment according to the invention includes a method of selecting a memory cell from a plurality of memory cells in a magnetoresistive random access memory (MRAM). The method includes passing current through a bit line corresponding to the memory cell, allowing current to pass through a word row line that is electrically coupled to a word line of the memory cell, and allowing current to pass through a word column line that is electrically coupled to a column line of the memory cell such that a word current passes through the word line of the memory cell.
Another embodiment according to the invention includes a method of producing a memory cell. The method includes forming a bit line with a body of the memory cell that can store a magnetic orientation in a hard layer to represent a logic state. The method further includes forming a word line that wraps around the bit line by at least 270 degrees and connects to a word row line and a word column line.